10 - Ohmic contacts to gallium oxide

Abstract

The deposition of a metal onto a semiconductor surface to provide low contact resistance, high-reliability electrical contacts without adversely affecting the device during the metallization process is one of the most important challenges in device fabrication. Consequently, a fundamental understanding of how contacts work is essential for successful device manufacturing and commercialization. The physics of carrier transport across the metal-semiconductor junction renders metal contacts either rectifying (a.k.a. Schottky) or nonrectifying. A nonrectifying contact whose relationship between current and voltage has a low interfacial contact resistance Rc, and is preferably linear, is referred to as an Ohmic contact. Achieving low contact resistance Rc (Ω mm) or contact resistivity ρc (Ω cm2) has required a great amount of investigation for every relevant semiconductor material in the past. Typically, the successful formation of an Ohmic contact has relied on three constituent requirements: highly or degenerately doped semiconductor, choice of metallization, and thermal annealing. Recent advances in bulk growth of single-crystal β-Ga2O3 have changed how this material is regarded in the device community and have brought it into the spotlight as an emerging wide bandgap semiconductor with potential for radio frequency (rf) and power electronics applications. The academic study of the mechanism of forming Ohmic contacts to Ga2O3 has been a less pressing topic while fundamental issues in growth and devices have been addressed. A sufficient body of literature to date has established a preliminary understanding of how Ohmic contacts to Ga2O3 work. In this chapter, we review the fundamentals of Ohmic contact operation, review the existing body of literature on Ohmic contacts to Ga2O3, and recommend future directions of research.